The present invention relates to pulse width modulator controllers designed for switching power supplies and, more particularly, to a switching regulator designed for low power applications.
Pulse width modulator controllers for controlling switching power supplies are well known. In general, such controllers provide output pulses that are used to drive a semiconductor switch. The semiconductor switch is therefore switched on and off at the frequency of the pulses. The semiconductor switch can be connected in series with the primary of a transformer to which is applied a DC input voltage. During the time that the semiconductor switch is on current flows through the primary to induce current flow and a voltage to be developed across the secondary of the transformer. The output voltage developed across the secondary can be rectified and filtered to provide, for example, a low voltage power source to some load utilization means as understood.
A typical pulse width modulator controller comprises modulator control circuitry including a sawtooth oscillator coupled to a flip-flop for developing, in conjunction with logic and drive circuitry, output pulses to control the operation of the semiconductor switch. A pulse width modulator comparator and error amplifier are included in the modulator control circuitry which function together to provide means to adjust the output pulse width in response to varying load demand by receiving feedback inputs as is understood. Thus, after initiation of each output pulse, the output of the comparator is caused to switch to a level state after a predetermined interval for terminating the pulse. By controlling the time delay between the initiation of the output pulses and the switching of the output of the comparator in accordance with variations in the load power demand the output voltage is regulated.
Prior art controllers of the type described above work quite well when driven from a low impedance power source capable of suppling sufficient current thereto to meet load power demands. However, a problem can arise when such pulse width modulator controllers are operated in low power applications and are operated from a power source having a relatively high source impedance. An example of such an application is in low power digital telephones wherein the input power source to the telephone instrument is the battery supply of the telephone exchange which is supplied to the telephone via the subscriber loop (telephone lines). The resistance of the subscriber loop can be very high depending on the distance that the telephone is from the telephone exchange.
In the above described application the amount of power available from the telephone exchange to the controller located in the telephone is limited due to the high source impedance associated with the telephone lines. If the load power demand of the telephone circuitry increases beyond the power available, the input voltage supplied to the controller from the telephone exchange can decrease thereby further reducing the input power supplied thereto. It is possible in at least some prior art controllers used in this low power application to become latched to operate in this positive feedback mode with low input power being supplied thereto and low output power to the load until the latter is completely disconnected which is undesirable.
Hence, a need exists for an improved pulse width modulation controller suited for low power applications which does not suffer from the latching problems associated with prior art controllers.